In general, the tire is formed of a plurality of rubber parts. Typically, as shown in FIG. 15, an inner liner rubber part 2, a tread rubber part 3, a side wall rubber part 4 and a rim strip rubber part 5, and the other part if any are formed of respective rubber materials or parts corresponding to respectively required property, when to form such rubber part, a die in conformity with the sectional shape of each of the rubber parts is used in an extruder for continuously extruding each of the rubber parts. Subsequently, the desired rubber part is obtained by cutting the extruded article in a given length. The tire is built by sequentially bonding the aforesaid rubber parts to one another on a rotary support body such as a tire-building drum.
More recently, however, precision requirements for tire configuration or profile have become stricter. In addition, there is a problem associated with strain or shrinkage of the rubber parts, which results from the process of continuously extruding the article from the extruder and cutting the article in the given length. Hence, as disclosed in the Patent Documents 1 and 2 listed later, it has become the practice to form the rubber part having a predetermined sectional shape by spirally winding an un-vulcanized rubber strip on the rotary support body such as a building drum along the circumferential direction of tire and in an overlapped relation, the rubber strip extruded in the form of a ribbon.
The present applicant has also proposed to regulate a positional shift or misalignment between a winding start position and a winding end position of the rubber strip to the range of zero to 5 mm with respect to the circumferential direction of the tire, when the aforesaid ribbon-shaped rubber strip is used to build the rubber part is used. By this way, improvement is made to the weight balance and the like of the tire. When the positional shift or misalignment become large, weight balance and uniformity of the tire deteriorated as to adversely affect running performance of the tire (later-listed Patent Document 3).
Meanwhile, if the amount of side-by-side overlapping of the aforesaid rubber strip is too great, there is a fear that the weight balance or uniformity of the tire may be adversely affected because the resultant rubber part suffers such a heavy unevenness or ruggedness of the sectional shape thereof as to fail to present the desired sectional shape. For this reason, the present applicant has also proposed to build the rubber part by winding the rubber strip in a manner to ensure that the amount of overlap with respect to the tire width is limited to the range of ½ to ⅕ of the width of the aforesaid rubber strip (later-listed Patent Document 4).
According to the above building method, the rubber-strip has a sectional shape substantially of a crescent, triangle or trapezoid which is decreased in thickness at edge portions on both sides, which are to be overlapped, in order to eliminate rugged or stepped portions after the rubber strip is wound and attached in manner side-by-side-wise partially overlapped.
As shown in FIG. 16, the rubber strip is formed as extruded from an extruder 110 that has a discharge port 117 in a sectional shape of substantially flatten crescent or triangle for example; the discharge port 117 being at distal end of a cavity 115a of the die 115 that is provided on head 113 of the extruder 110.
AS shown in FIGS. 17-19, the die 115 being used for such molding has a shoulder 118 for narrowing down at the distal end of the cavity 115a while the discharge port 117 is located at middle of the shoulder 118. The cavity 115a is generally shaped as tapered as gradually narrowed toward the shoulder 118 from rear-end opening 119 of the die, which is connected with the head 110. The molten rubber being forwarded to the die 115 is given further enhanced pressure by narrow-down effect of the shoulder 118.
For partial overlapping of thus extruded rubber strip material at winding-wise attaching at forming of tire, it is more preferred that transversely opposite near-edge parts of the strips have smaller thickness. Nevertheless, this causes a great difference in flow velocity and pressure for the molten rubber in the die 115 between a center portion and transversely opposite near-edge parts. Therefore, with decrease in the thickness at the near-edge parts, the rubber strip is more likely to suffer defects such as cracks and cutouts at the transversely opposite near-edge parts.
The building methods disclosed in the Patent Documents 3 and 4, in particular, suggest that the rubber strip may preferably have the sectional shape substantially of a relatively flat crescent having the width of 5 to 30 mm, the thickness of 0.5 to 3.0 mm at around centerline, and the thickness of 0.05 to 0.2 mm at along the edges on transversely opposite sides. The only way to be taken by these methods to reduce the thickness at along the edges to 0.05 to 0.2 mm is to extrude the rubber strip from the dies of the extruder at a lower speed thereby to apply tension to the extruded rubber strip for elongation. As a result, the defects such as cracks and cutouts are more likely to occur at portions along edges having reduced thickness.
In the tire manufacturing, a cycle time for building each tire is a crucial factor. Unfortunately, the aforementioned configuration of the cavity of the conventional dies entails the problem that there is the great flow rate difference of the rubber material between the central part and the near-edge parts on opposite sides, in the cavity. Thus, if the rubber strip has an increased width, it is difficult to achieve a desired thickness at along edges of the rubber strip. If, on the other hand, the rubber strip has a narrow width, the thicknesses at along edges become large. Hence, the only way to satisfy the requirements that the rubber strip is increased in width but decreased in thickness at along the edges is to form the rubber scrip at a lowered extruding speed. Consequently, it is unavoidable to impair the building cycle time of the tire manufacturing.
Most of the aforementioned problems are derived from the variations in viscoelastic properties of rubber material being extruded from the extruder and the flow rate differences of the rubber material in the cavity of the dies. In view of use of such rubber material, a solution to the aforementioned problems is desired. Particularly, a measure for satisfying the requirement of reducing the thickness at along edges of the rubber strip (0.05 to 0.2 mm) not only offers a significant contribution to the reduction of vulcanizing time of tire but also provides an effective means for reducing defects such as bareness and air inclusion occurring at the surface of the vulcanized tire. Hence, the solution to the above problems is strongly desired.
Patent Document 1: JP-A-2000-202921
Patent Document 2: JP-A-9(1997)-029858
Parent Document 3: JP-A-2002-178415
Patent Document 4: JP-A-2002-205512